1. Field of the Invention
The present invention relates to a polishing slurry, and particularly, a polishing slurry used for polishing in the wiring-formation process of a semiconductor device, and a polishing method using the polishing slurry.
2. Description of the Related Art
Recently, as a Large-Scale Integrated circuit (hereinafter, referred to as “LSI”) of a semiconductor advances to fulfil the demand of higher integration and performance, new microprocessing techniques have been developed. The Chemical Mechanical Polishing (hereinafter, referred to as “CMP”) method is one of the new microprocessing techniques, which is often used in a LSI production process, especially for flattening interlaminar insulating films, formation of a metal plug and embedded wires in a multi-layered wiring-formation process. This technique is disclosed in U.S. Pat. No. 4,944,836.
Additionally, in recent years, use of copper and copper alloy as a wire material has been mainly attempted in order to enhance the performance of LSI. However, in the case of the copper and the copper alloy, it is difficult to perform microprocessing according to dry etching, which has been often used in the conventional method for forming aluminum alloy wiring. Therefore, there has been employed what is called Damascene method in which copper or a thin film of copper alloy is accumulated on and embedded in an insulating film by way of a groove formed in advance on the insulating film and then the copper or the copper alloy thin film remaining at the portions other than the groove portion is removed by CMP, whereby embedded wiring is formed. For instance, this technique is disclosed in Japanese Patent Application Laid-Open (JP-A) No. 2-278822.
In general, a CMP method in metal processing such as copper and a copper alloy includes the steps of: sticking a polishing pad on a disc-shaped polishing platen; soaking the surface of the polishing pad with a polishing liquid for metal; pressing, one surface of a substrate on which a metal film has been formed, against the surface of the polishing pad, with applying a predetermined pressure (hereinafter, referred to as “polishing pressure”) thereon from the back surface of the substrate, and rotating the polishing platen in that state; and removing the projected portion of the metal film by utilizing mechanical friction between the polishing liquid and the projected portion of the metal film.
The polishing liquid for metal used for CMP generally contains a metal-oxidizing agent and solid polishing particles. An oxidized metal dissolving agent and a metal anticorrosive agent are further added thereto, according to necessity. With regards to the basic mechanism of CMP, it is assumed that at first the surface of the metal film is oxidized by an oxidizing agent; and then the oxidized layer is scraped off by the solid polishing particles. The oxidized layer existing at the dented portion of the metal surface is hardly brought into contact with the polishing pad and thus the solid polishing particles do not have so much scraping effect thereon. In other words, the metal layer present at the projected portion of the substrate surface is removed as CMP is effected, whereby the substrate surface is made flat and smooth. The details of the feature described above is disclosed in Journal of Electrochemical Society, vol. 138, No. 11 (1991), pp. 3460-3464.
It has been known that addition of an oxidized metal dissolving agent is effective as a method for increasing the polishing rate by CMP. It is understood that addition of an oxidized metal dissolving agent is effective because the scraping effect by the solid polishing particles is enhanced by dissolving (hereinafter, referred to as “etching”) the particles of metal oxides, which have been scraped off by the solid polishing particles, in the polishing liquid. The polishing rate due to CMP is increased by adding the oxidized metal dissolving agent. However, if the oxidized layer existing at the dented portion of the metal film surface is also etched and the metal film surface is exposed, the metal film surface is further oxidized by the oxidizing agent. In a case in which such excessive etching is repeated, the metal film at the dented portion is considerably etched. Thereby, a phenomenon (hereinafter, referred to as “dishing”) is occurred in which the center portion at the surface of the embedded metal wiring is recessed like a dish after polishing, and the flattening effect is damaged.
In order to prevent such excessive etching from being occurred, a metal anticorrosive agent for protecting the metal surface further is added. The metal anticorrosive agent forms a protective film on the oxidized layer of the metal film surface, and prevents the oxidized layer from solving in the polishing slurry. This protective film can be scraped off easily by the solid polishing particles, and the polishing rate due to CMP is preferably maintained.
In order to suppress dishing and etching of copper and copper alloy during polishing and form highly reliable LSI wiring, there has been proposed a method for using a polishing liquid for metal containing: an oxidized metal dissolving agent composed of an amino acetic acid (such as glycine) or amido sulfuric acid; and BTA (benzotriazol) as the metal anticorrosive agent. This technique is disclosed, for example, in JP-A No. 8-83780.
In the Damascene wiring formation of copper or copper alloy or the metal-embedding formation such as plug wiring formation of tungsten or the like, if the polishing rate of the silicon dioxide film as the interlaminar insulating film formed at the portion other than the embedded portion is also large, there arises the phenomenon of “thinning” in which reduction of wiring thickness and reduction of thickness of the interlaminar insulating film simultaneously occur. As a result, since there is generated variation in resistance due to the increase in the wiring resistance, the pattern density or the like, a characteristic that the polishing rate of the silicon dioxide film is sufficiently smaller than that of the metal film to be polished is required. Therefore, there has been proposed a method in which pH of the polishing liquid is made higher than the negative value of the logarithm of the dissociation constant Ka (pKa) of the first dissociable acid group of an oxidized metal dissolving agent −0.5 by suppressing the polishing rate of silicon dioxide by the action of an anion produced as a result of dissociation of an acid. This technique is disclosed, for example, in Japanese Patent No.2819196.
As a lower layer beneath metal for wiring portion such as copper or copper alloy, a barrier layer of a conductor selected from the group, for example, consisting of tantalum, tantalum alloy, tantalum nitride and tantalum compounds of other types (hereinafter, referred to as “tantalums”) is formed in order to prevent copper from diffusing into the interlaminar insulating film and enhance the adhesion. Accordingly, at the portion other than the wiring portion at which copper or copper alloy is to be embedded, the exposed barrier layer must be removed by CMP. However, as the barrier layer conductor is harder than copper or copper alloy, a sufficiently high polishing rate is hardly obtained and flatness often worsens, even if a combination of the polishing materials for copper or copper alloy is simply employed. Therefore, there has been studied a two-stage polishing method including the first step of polishing metal for wiring portion and the second step of polishing the barrier layer conductor.
The conductors generally used as the barrier layer, for example, the above tantalums, titanium and the compound thereof, tungsten and the compound thereof or the like are chemically stable. In addition the etching of the conductors is difficult, and the conductors have high hardness. Thereby, the mechanical polishing thereof is not so easy as that of copper and a copper alloy. When the hardness of the polishing particles is increased, polishing crack is occurred on copper or copper alloy, and the poor electrical property is caused. When the particle concentration of the polishing particles is increased, a problem exists in that the polishing rate of a silicon dioxide film becomes faster, and thinning is occurred.
It has been considered that in CMP of the barrier layer as the second process, the dishing in the embedding wiring portion of copper or copper alloy must be prevented and the low pH of the polishing slurry is minus effect so as to suppress the polishing rate and etching rate of copper or copper alloy. On the other hand, as the polishing slurry which is effective for polishing the barrier layer, the following polishing slurry is proposed. The pH of the polishing slurry is 3 or less and the concentration of the metal-oxidizing agent is within the range of 0.01 to 3.0 percent by weight by adjusting the pH of the polishing slurry and the concentration of the metal-oxidizing agent such that the polishing of tantalums used as the barrier layer conductor is easily advanced in a low pH region and in a low metal-oxidizing agent concentration region (For example, see WO 01/013417).
However, the polishing rate of the barrier layer due to the polishing slurry proposed above is not sufficient as compared with the etching rate and polishing rate of metal for wiring such as copper and copper alloy. A problem exists in that the state (hereinafter, referred to as “corrosion”) where a foreign matter is occurred on the oxidized surface to be polished is easily occurred on the metal surface, particularly on the metal for wiring such as copper and copper alloy.
The present inventors have found that the high polishing rate of the conductor used for the above barrier layer in a region where the pH is 3 or more is obtained for the above proposal. At that time, the present inventors have also found that as the kind of the oxidized metal dissolving agent used for pH adjustment, an acid in which pKa of a first dissociable acid group is 3.5 or more and an ammonium salt of the acid are effective.
Since the corrosion operations of the metal for wiring such as copper and copper alloy and the conductor used for the barrier layer are respectively small in a region wherein the pH is 3 or more in the invention, the corrosion of the metal wiring portion is not easily occurred, and the highly reliable LSI wiring can be formed. In addition, in the region where the pH is 3 or more, the etching rate and polishing rate of the metal of the wiring portion can be sufficiently reduced as compared with the polishing rate of the barrier layer, and thereby, dishing or thinning is effectively reduced. The concentration of the metal anticorrosive agent to the metal surface can be also reduced.
It is an object of the invention to provide a polishing slurry which can realize the high polishing rate of the conductor used for the barrier layer in low polishing particle concentration, and can form the embedding pattern of a highly reliable metal film by suppressing the etching operation of the metal for wiring such as copper and copper alloy by reducing the dishing and thinning of the metal wiring. It is another object of the invention to provide a polishing method using the polishing slurry.